Ultrafast multiphoton ionization of Xe in strong extreme ultraviolet free-electron laser (FEL) fields (91 eV, 30 fs, $1.6\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$) has been investigated by multielectron-ion coincidence spectroscopy. The electron spectra recorded in coincidence with ${\mathrm{Xe}}^{4+}$ show characteristic features associated with two-photon absorption to the $4d^{-2}$ double core-hole (DCH) states and subsequent Auger decay. It is found that the pathway via the DCH states, which has eluded clear identification in previous studies, makes a large contribution to the multiple ionization, despite the long FEL pulse duration compared with the lifetime of the $4d$ core-hole states.

• Received 17 December 2019
• Accepted 14 April 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.193201